Various methods have been proposed to form three-dimensional articles by depositing of layers of material on a substrate. This layering manufacturing is also known as solid free-form fabrication or rapid prototyping. A computer model of a desired object is sliced into a finite set of layers. The layers are created sequentially, or bonded onto a previously formed layer. This layering creates an object which approximates the intended geometry of the three-dimensional object, in a much as the layers cause a "staircase effect" at the edge or peripheral area of the object. The staircase effect is an artifact of the process of applying the discrete layers of material. The final appearance of the object can be improved by minimizing the layer thickness or by using such additional processing steps as sand blasting and the like which smooth out the surface of the object.
Stereolithography is one method for forming a three-dimensional polymeric article made of a polymeric material. In stereolithography, a photopolymer is selectively cured using a laser beam to create each layer. The three-dimensional article is built up on an elevator-type platform in a vat containing the liquid photopolymer. Successive layers are created by lowering the partially created objects into the photopolymer liquid and laser curing a new layer of photopolymer material on the top of the partially built object.
Another method comprises a fused-deposition modeling process which melts and extrudes a polymer substance through a nozzle onto a target to form the three-dimensional object.
Still another method involves lamination wherein layers of a paper or polymer material are cut and bonded to the substrate, then trimmed on the edges or peripheries with a laser to correspond to a desired layer or cross-section through the article. The unwanted or waste areas of each layer are cut into a grid. These "squares" stack up to form square prisms, truncated by the boundary of the object. The "square" areas are physically removed after product completion, leaving only the desired part.
Still another method uses a lamination process wherein papers or other mask-type materials are used to build layers of material. A laser beam cuts the layer geometry into the paper which is used as a mask. The material is deposited in the cutout area which defines a single layer of the three-dimensional article and portions of the material overlaps onto the mask material.
In the past, it has been difficult to form three-dimensional articles made of a metal by using a free-form fabrication or deposition layering process.
One metal fabrication method involves a laser sintering process which spreads a layer of a metal powder material on top of partial objects and then selectively sinters (using a laser beam) the portion which comprises the new layer.
Another metal fabrication method involves a post sintering process wherein metal powder materials are bonded together with a polymer binding material. However, it is difficult to entirely remove the polymer binding material from the finished three-dimensional object. The presence of binder residue detracts from the desired strength and other properties of the metal object. In addition, removal of the polymer material causes voids in the three-dimensional object such that the object is somewhat porous. Another metal material (such as a lower temperature metal) can be infiltrated into the porous three-dimensional object with temperature differentiation, in an attempt to fill the voids. However, the three-dimensional object then has a "honey-comb" type composite structure of less than desirable properties and is subject to creeping or warping during sintering of the original host material. In addition, the presence of residual polymer and/or the fill material act as contaminates within the three-dimensional object and thereby affect the properties of the object. The contaminates may include products of oxidation, excess carbon, binder residue and the like. It is to be understood that the use of filling or fiber elements in the infiltration process is different from the use of alloy materials. In infiltration the two materials remain distinct; whereas, in an alloy the materials are homogeneously blended together to achieve a desirable combination of properties. Another concern is that whenever the infiltrated material bonds imperfectly with the matrix material, the microstructure has a very large number of stress concentrators, thereby reducing the strength of the object. While such three-dimensional "infiltrated" objects are sometimes called "fully dense" objects, such a term is misdescriptive of the actual characteristics of the three-dimensional object since the three-dimensional object is not comprised of substantially one type of a preferred metal.
Still other fabrication methods use metal deposition techniques in conjunction with a metal removal technique such as milling, grinding, sand blasting and the like. The "staircase" effect and the roughness at the edge of each layer are eliminated by machining each layer and its peripheries after the layer is deposited. It is the machining or metal removal process that determines the actual dimensional accuracy of the three-dimensional object.
Currently there are several methods and apparatuses for depositing molten material. For example, the Mertz et al. U.S. Pat. No. 5,281,789 describes a welding process and an apparatus for depositing molten metal. A molten metal is deposited on a work surface and subsequent layers of metal are deposited thereon. An electrode and weld torch are preferably movable as a unit so that the molten metal may be deposited onto selective locations on the work surface. Alternatively, the work surface may be moveable while the weld torch and the collector electrode are moveable or held stationary so as to selectively position the deposited material on the work surface. The droplet size is controlled by applying additional mechanical energy to the feed wire to constantly vibrate the feed metal.
The Prinz et al. U.S. Pat. No. 5,286,573 describes a method using support structures for the creation of objects by a layer deposition process. In the deposition process, each layer is composed of two portions. One portion represents a cross-sectional slice of a three-dimensional object being built ("the object") and is composed of the desired deposition material or materials. The other portion is the complement of the object shape of the first portion and serves as a support structure which supports the growing object form ("the support"). The object material and the support structure material are each applied in a predetermined sequence. A plurality of layers, each placed upon the previous layer, is formed. In this way, a layered structure is built up. The layered structure contains the object made of the deposition material surrounded by the support material. For each layer, both, or one of, or neither of the support material and the object material can be shaped to produce its desired object. Preferably, the shaping occurs after the object or support material is applied and before the subsequent layer is applied.
The Prinz et al. U.S. Pat. No. 5,301,863 describes an automated system having multiple work stations for forming objects by incremental buildup of layers. Each layer a cross-sectional slice of a three-dimensional object being built and is composed of the desired object material. In addition to the object material, each layer usually also contains a second portion that acts as a complement of the object shape of the deposition material portion and serves as a support structure for the growing object form. During the manufacture of the article, several operations are performed on the workpiece for each layer. In addition to the material deposition station, a plurality of processing stations are employed, each of which has at least one separate function. These functions can include any combination of shot peening, cleaning, blasting, heat treating, shaping, inspection, mask making and packaging.
The Prinz et al. U.S. Pat. No. 5,301,415 describes a method for the fabrication of three-dimensional articles by incremental material buildup of layers of material. In one embodiment, a layer of object and support material is applied. Depending on the shape of the object, either one or the other material is applied first, then shaped to achieve dimensional accuracy, and then the other material is deposited. The deposited layer is then machined, cleaned, shot-peened and the like. The process is repeated until all layers have been placed. After the final layer has been applied, the complementary material is removed leaving the created object formed of the deposition material.
The deAngelis U.S. Pat. No. 5,398,193 describes a method and an apparatus for making a three-dimensional object through controlled layer-by-layer deposition and/or extraction. A three-dimensional computer model representation of the three-dimensional part is provided and the model representation is sliced into a plurality of successive layers corresponding to layers of predetermined thicknesses of the part. The computer model generates sequences of the part and any complementary support material contours which correspond to each layer. Materials for one or more contours are deposited onto a work surface within a processing enclosure. Portions of the material are removed from the contours. The deposition processing and removing steps are repeated as necessary under the control of the computer model to complete the three-dimensional object. Further processing includes machining off a sublayer to ensure thickness tolerances or roughening and chemically enhancing the surface to ensure selective binding to the next aggregate layer. The controlled layer creation steps are repeated to build the entire part surrounded by complementary materials which are then removed to obtain a fabricated part.
A major disadvantage of the above methods is that the machining portion of these methods is relied upon to achieve the desired dimensional accuracy of the three-dimensional object. In many situations, the objects being formed require multiple post fabrication steps to produce an acceptable three-dimensional object or end product.
There is a need for an improved method for creating three-dimensional or solid objects which utilizes accurate deposition of the material onto a work surface or substrate. However, until the present invention, there has been no disclosure or suggestion that a supply of droplets could be accurately controlled and dispensed to form a high quality three-dimensional or solid article in a net shape without the use of a collector or mold.
One method for forming a spray of substantially uniform size droplets is disclosed in the Chun et al. U.S. Pat. No. 5,266,098 which describes a process and an apparatus for producing and maintaining charged, uniformly sized metal droplets. The droplets are deposited as a spray to coat a substrate. A droplet generator is disposed within a spray chamber. The droplet generator comprises a container for holding and liquefying a charge of metal, a means for forming uniformly sized metal droplets, and a means for charging the metal droplets as the droplets are formed. The forming means is preferably either a vibrating means for vibrating the molten metal in the container (or at least one oscillating gas jet disposed outside the container at the point where the liquified metal exits the container). The liquified metal is forced from the crucible through an orifice in the container so as to form the metal droplets. As the liquified metal exits at least one orifice as a jet or stream, the imposed vibrations in the liquified metal cause the jet to break up into uniformly sized metal droplets. An electrical charge is applied to the droplets as the droplets are being formed. The metal droplets may be charged by either charging the liquified metal while in the container or by charging the droplets as, or after, the droplets are formed after exiting the crucible. As each droplet breaks from the jet or stream, the droplet retains a portion of the charge. With that charge, the droplets repel each other in flight and scatter into a cone-shape as the droplets fall toward a substrate. When the uniformly sized droplets are charged, the droplets are oriented to form a cone configuration due to the like polarity of the droplets and the repelling of each droplet from its neighboring droplet. The Chun et al. '098 patent further claims the application of an electric field in the flow path of the metal droplets to change their trajectories.
A thesis submitted by C.H. Passow to the Department of Mechanical Engineering at the Massachusetts Institute of Technology (MIT) on May 5, 1992 describes a study of spray forming using uniform droplets sprays, droplet placement production techniques, and droplets selection and deflection wherein parallel plates are positioned below the charging plate to deflect the charged droplets off to the side where they would be collected. Uncharged droplets would pass unhindered.
An article by P.J. Acquaviva et al. entitled Issues in Application of Thermo Spraying to Melt Mold Fabrication published in IBEC International, 1994, describes a uniform droplet spray and deposition process which can be manipulated by moving a substrate at various speeds and directions.
A thesis submitted by Godard Karl Abel to the Department of Mechanical Engineering at MIT on May 18, 1994 describes using a uniform droplet spray forming process to form deposits on stationary and moving substrates; the spray forming of three-dimensional parts; and, instead of allowing droplets to scatter randomly based on an unknown disturbance, the droplets could be charged to varying amounts and then deflected to create a more predictable mass flux distribution.
The Orme et al. U.S. Pat. Nos. 5,171,360; 5,226,948; 5,259,593; and 5,340,090 describe methods and apparatuses for forming a net form product by directing a stream of a liquid material onto a collector of the shape of the desired product. A time variable disturbance is applied to the stream to produce a liquid droplet stream with the droplets impacting on the collector and solidifying into a unitary shape. The Orme et al. 1995 paper presented at SFF in Austin, Tex. describes thermal design parameters for the development of solid free-form fabrication of structural materials with controlled droplets.
In view of the need for a better and more efficient method for the manufacturing and forming of three-dimensional solid objects, and as a result of extensive research, a new method for creating a three-dimensional solid object by depositing a molten metal has now been developed.
As far as is known, there is no disclosure that a three-dimensional solid object can be formed by dispensing uniformly sized metal droplets incrementally in layers in a highly accurate manner.
Accordingly, it is an object of the present invention to develop an apparatus and process for manufacturing high quality solid metal objects. The present invention further provides a process which does not involve the use of multiple processing steps to form each layer of deposition, or otherwise achieve dimensional accuracy of the three-dimensional solid object.